This invention relates to floating gate memory devices, and more particularly to methods of programming such memory devices. The invention is pertinent to floating gate devices with separately optimized read and programming transistors coupled by a common floating gate, and which further are programmed by gate currents in the programming-optimized transistor. See, for example, B. Eitan et al. "Hot-Electron Injection into the Oxide in n-Channel MOS Devices", IEEE Transactions on Electron Devices, Vol. ED-28, No. 3, Mar. 1981, pp. 328-40. The invention will be fully understood from the following explanation of it in the context of metal oxide semiconductor ("MOS") erasable programmable read-only memory ("EPROM") devices.
A typical MOS EPROM device which is programmable in accordance with this invention has a control gate (sometimes referred to simply as the gate), a floating gate, a source and drain pair for programming the device, and a source and drain pair for reading the programmed state of the device. The unprogrammed initial voltage condition of the floating gate is for the capacitance between the floating gate and the source to be relaxed (i.e., 0 volts) when the gate voltage Vcg=0. This initial condition is important because in a capacitive divider such as shown in FIG. 3 the potential of the floating gate Vfg is given by the equation Vfg=Vfg(initial)+(C1/(C1+C2))(Vcg-Vcg(initial)).
The device is conventionally programmed by applying a relatively high voltage (e.g., 15 volts) to the gate and a somewhat lower voltage (e.g., 8 volts) to the programming drain, while holding the programming source at ground potential (i.e., 0 volts). Capacitive coupling between the control gate and the floating gate causes a change in the potential of the floating gate when the potential of the control gate is changed. For example, if the capacitive divide or coupling ratio is assumed to be 0.7 and both the control gate and the floating gate are initially at 0 volts, then raising the control gate to 15 volts causes the floating gate potential to rise to 10.5 volts. The programming transistor's channel begins to conduct a substantial current. Due to the very high field in the drain region, electron hole pairs are formed with a very high kinetic energy. Some of the electrons will have enough kinetic energy to be injected onto the floating gate where they are trapped. This in turn lowers the potential of the floating gate (e.g., to approximately 6.5 volts in the example being discussed), thereby "programming" the device.
When the control gate potential is lowered from 15 volts to 0 volts, the capacitive divider causes a 10.5 volt drop in potential from 6.5 volts to -4 volts on the floating gate. Because the floating gate transistor is an NMOS transistor with a threshold voltage of about 1 volt, the floating gate transistor is non-conducting. Furthermore, when the control gate is raised to 5 volts, there results a 3.5 volt rise on the floating gate to -0.5 voltage. The programmed transistor therefore remains off. In other words, the read channel of a programmed device is always off regardless of the normal logic level (0 or 5 volts) applied to the gate of the device. In an unprogrammed device (i.e., no negative charge on the floating gate), the read channel of the device is on or off depending on whether 5 volts or 0 volts, respectively, is applied to the gate of the device because the floating gate potential is respectively 3.5 volts (above the threshold) or 0 volts (below the threshold).
As floating gate devices are made smaller, their ability to withstand the relatively high voltages (e.g., the above-mentioned 15 volts) conventionally required to program them decreases. Accordingly, the ability to withstand these voltages has become a limitation on further device size reduction which would otherwise be possible and very advantageous.
It is therefore, an object of this invention to reduce the voltages required to program floating gate memory devices.
It is another object of this invention to reduce the time required to program floating gate memory devices.